The present invention relates to an X-ray examination apparatus, including an X-ray source for emitting an X-ray beam, an X-ray detector for detecting an X-ray image and converting it into an optical image, and a video extractor which is coupled to the X-ray detector via optical coupling means, the optical coupling means being provided with an optical pick up for feeding a fraction of the light flux to a photosensor which produces a control signal for adjusting the X-ray flux from the X-ray source.
Such an apparatus is known from the International patent application WO 96/20579-A1. The cited document describes an X-ray examination apparatus with an exposure control circuit which supplies a control signal for adjustment of an X-ray source. In case of strongly overexposed areas within a measuring field selected in an X-ray image, the X-ray flux from the X-ray source is reduced by means of said control circuit. In a specific embodiment the signal applied to the exposure control circuit is obtained by means of an optical pick up and a photodetector. The optical pick up and the photodetector, inserted in the collimated beam of the optical coupling means, form a linearly responsive detection unit which carries out the photosensing for X-ray control purposes. As the incorporated photodetector integrates the detected light flux over a selected measuring field, an average value for the X-ray detector output luminance will be found after amplification of the photodetector current. The problem arising in the control loop for the X-ray source is caused highlights, particularly direct radiation on the X-ray detector. As these highlights change in respect of area and amplitude, the control signal from the measuring field is greatly influenced by peak values that may reach values as much as 100 times larger than the signals of interest. The resultant image will be underexposed and relevant image information may be lost.
Depending on the specific applications, a dose from 1 nGy to 10 .mu.Gy is applied. As already mentioned, with highlights and direct radiation the signal to be detected may vary with a factor 100, so that the photosensor must be sensitive for a signal amplitude range of 5 decades. The dynamic range of the known linear detection unit is then insufficient for pixelized sensors, e.g. CCDs.
Many proposals have been made to cope with this problem, particularly the application of sensor systems in which anatomically determined measuring fields are selected and which are provided with some intelligence to mitigate the consequences of highlights and black areas. In other proposals the optical image obtained by means of a fraction of the X-ray signal is analyzed; on the basis of these analyses the adverse effects of black and extreme white areas are suppressed, after which the examination is performed with full luminance. All of such systems are based on linear detection on a pixel basis; they all involve black and extreme white exclusion in a more or less intelligent way. The detectors are based on a spatially sampled system, i.e. on the basis of pixels, with CCDs.